In one preferred embodiment, the present invention relates to an improved method of and apparatus for depositing thin films, such as electrically conductive, light transmissive films, barrier layer films, passivating films, etc., via a process which is easy to control and which avoids damage to the substrate from the depositing species. In particular, the deposition of such thin films represents one step in the manufacture of amorphous semiconductor devices, such as photoreceptive and photovoltaic devices. Furthermore, the instant invention allows for reliable control of the deposition process, through a wide range of previously unattainable deposition rates. Other preferred embodiments of the present invention, disclosed herein, deal with the deposition of thin films onto the surface of metal, glass, plastic, or even low melting point substrates.
Vacuum evaporation techniques represent a first attractive process for the fabrication of the thin film layers such as those described hereinabove, insofar as vacuum evaporation technology is simple to implement, economical, and may be performed at high deposition rates. However, such techniques have several limitations which have heretofore prevented the use thereof in a still wider variety of applications. In conventional vacuum evaporation processes, the material to be deposited is heated in a vacuum, thereby vaporizing same. The vaporized material is then condensed onto a substrate maintained in close proximity thereto, thus forming a thin film. Full utilization of this technique is limited by the facts that (1) many of the materials employed to deposit thin films have relatively low vapor pressures, thus requiring high temperatures for the evaporation thereof, and (2) many precursor compounds (the material to be deposited) decompose or disproportionate when evaporated, thereby resulting in the deposition of thin films having a composition differing from that of the evaporated precursor compounds. For these reasons, reactive evaporation processes, and especially activated reactive evaporation processes, are favored for the preparation of specially configured thin films fabricated from specific precursor compounds.
In reactive evaporation processes, thermally generated vapors of the precursor material to be deposited, which vapors emanate from a source of the precursor material, such as a heated crucible, react with the residual atmosphere specifically maintained in an evaporation apparatus to produce a molecular species which is then condensed onto a proximately disposed substrate so as to deposit a thin film layer thereupon. In this manner, the amount of disproportionation is limited, and vapors of difficult to evaporate materials may be generated. For example, a mixture of indium metal and tin metal may be heated at reduced pressure in an evaporation apparatus having a specifically maintained residual atmosphere of oxygen therein to condense a thin film of indium tin oxide onto a proximately disposed substrate. The deposition of said indium tin oxide has apparent utility as a transparent electrical conductor for semiconductor devices. Similarly, the vapors of titanium metal may be heated and reacted in a residual ammonia atmosphere to deposit a thin film of titanium nitride onto a substrate. However, major shortcomings of reactive evaporation processes exist due to the fact that the vapor phase reaction rates of many precursor materials are relatively slow, and, consequently, processes which rely upon these reaction rates will exhibit inherently low deposition rates.
The reaction rate of the vaporized precursor material and the residual atmosphere of gas in the evaporation chamber may be accelerated through the use of an activated evaporation process. One such activated evaporation process is disclosed in U.S. Pat. No. 4,336,277 of Bunshah and Nath entitled "Transparent Electrical Conducting Films By Activated Reactive Evaporation"; and further described by Bunshah and Nath in a publication entitled "Preparation of In.sub.2 O.sub.3 and Tin-doped In.sub.2 O.sub.3 Films by a Novel Activated Reactive Evaporation Technique", published in THIN SOLID FILMS, Vol. 69 (1980). As disclosed in and taught by the foregoing patent and publication, resistive heating is employed to evaporate indium and tin metals, while an electron flux is provided from a thermionic emitter to generate the activating ionized plasma. However, in the disclosed process, pressures in the range of 10.sup.-4 torr are required to provide a sufficient number of metal atoms and gas atom collisions with the plasma ions to accomplish the requisite metals-oxygen reaction. Further, in the described Bunshah and Nath process it is essential that (1) an inert gas such as argon be introduced into and be primarily ionized within the plasma zone to aid in the secondary ionization of the oxygen atoms, and (2) a magnetic field be employed to move the ions through the plasma zone in a helical path for increasing the length of time the ions remain therein, thereby increasing the number of possible ion collisions with the oxygen atoms and metal atoms.
Another activated reactive evaporation process is disclosed in U.S. Pat. application Ser. No. 448,139 of Nath filed Dec. 9, 1982, and entitled "Apparatus For And Method Of Depositing A Highly Conductive, Highly Transmissive Film," which application is assigned to the assignee of the instant patent application, and the disclosure of which is incorporated herein by reference. In the Nath patent application, a resistance heated crucible is employed to generate vapors of a solid precursor metallic material, and a radio frequency generated plasma is employed to activate the metal vapors, such as indium and tin, and promote the reaction thereof with a gas, such as oxygen, so as to deposit a thin film of, in the preferred embodiment, indium tin oxide onto the substrate. It is to be noted that the method disclosed in the activated reactive evaporation process of Bunshah and Nath, and the method disclosed in the r.f. plasma activated evaporation process of Nath are limited to the use of resistance heated crucibles to generate vapors of the solid material. While many solid precursor materials may be successfully evaporated by means of resistance heating, other precursor materials have melting points sufficiently high that resistance heating: (1) fails to evaporate those precursor materials at all, (2) fails to produce enough vapors from those precursor materials to result in a practical rate of deposition, or (3) which would effectively vaporize the precursor materials would simultaneously deteriously affect, for instance, a low melting point substrate, or a body of semiconductor material deposited on a substrate.
In contrast thereto, electron beam heating may be advantageously employed to vaporize high melting point materials. In electron beam evaporation, an electron source, such as a thermionic emitter, provides a focused energetic beam of electrons which is directed by, for instance, an electromagnetic field, to impinge upon, and thereby heat the precursor material so as to effect the evaporation thereof. As a further desirable feature, note that electron beam evaporation is a process which is easily controlled, since the electron beam may be rapidly switched or modulated. Therefore, an electron beam evaporation process is not only specifically well suited for the evaporation of high melting point materials such as refractory materials, but also, because of the fine degree of control available, is quite useful for evaporating a wide variety of low melting point materials.
The utility of such electron beam initiated reactive evaporation processes for depositing thin films should thus be apparent from the foregoing discussion. One such process, currently in limited use, relies upon the ability of an energetic beam of electrons impinging upon the surface of the precursor material being evaporated to produce secondary electrons, that is to say, to eject electrons from the precursor material. It is thereby possible, by positioning a positively charged electrode in close proximity to the electron beam heated precursor material, to trap the secondary electrons in sufficient proximity to the vaporized precursor material that the secondary electrons will activate the vaporized material in the manner previously described. This type of activated reactive evaporation process is well known to those skilled in the art; see, for example, "Processes of the Activated Reactive Evaporation Type and Their Tribological Applications" published by Bunshah in THIN SOLID FILMS, Vol. 107, No. 1, Page 26 (1983). A noteworthy shortcoming of activated reactive evaporation processes which rely upon secondary electron emissions to activate the vapors is the dependency of those processes upon those secondary electrons, and those processes are therefore of limited utility. The formation of secondary electrons will depend upon the type of precursor material being evaporated, the temperature of the precursor material during the evaporation process, as well as the rate of evaporation of the precursor material. Low melting point precursor materials, such as the indium tin alloy commonly employed to form indium tin oxide films, evaporate at relatively low temperatures, and accordingly, provide only a limited number of secondary electrons from which to activate the vapors of the precursor materials. Further, even high melting point precursor materials, such as titanium and chromium, which have low evaporation rates, provide only a limited number of secondary electrons. For these reasons, known methods of activated reactive evaporation are not well adapted for (1) low rate depositions, or (2) use when low melting point precursor materials are being evaporated.
Accordingly, there still exists a need for an activated reactive evaporation process employing an electron beam to vaporize a source of precursor material, which process does not rely upon secondary electron emissions for the activation of the vaporized precursor material. According to the principles of the instant invention, radio frequency energy may be advantageously employed to form an activating ionized plasma in a reactive evaporation system. Radio frequency activation may be combined with an electron beam source to vaporize the precursor materials to provide an activated reactive evaporation system which is not dependent upon the emission of secondary electrons for the formation and maintenance of the activating plasma, and which is specifically adapted to deposit a wide variety of precursor materials at varying deposition rates.
U.S. Pat. Ser. No. 4,361,114 of Gurev, entitled, "Method And Apparatus For Forming Thin Film Oxide Layers Using Reactive Evaporation Techniques," discloses a system for the preparation of thin films of indium tin oxide materials. According to the method described by Gurev, oxygen, which is adapted to ultimately react with the vapors of indium and tin, is ionized by a radio frequency signal at its point of introduction into the system, thereby promoting the subsequent reaction thereof with the vaporized indium and tin. It is to be noted that in the system of Gurev, radio frequency energy is used to excite only the reactant gas, oxygen, at a point remote and isolated from the point at which the vapors of the solid precursor materials are generated. Accordingly, only the reactant gas is activated by the radio frequency energy field, and the reaction rates, and the subsequent deposition rates, are lower than they would be if both species were activated. Because the apparatus of Gurev does not provide for radio frequency excitation of the precursor materials evaporated therein, it cannot be utilized to promote the recombination of species of evaporated precursor materials which have disproportionated. Further, there is no reason present in that or other patents and literature for modifying the system of Gurev to use the radio frequency energy field to activate both the reactant gas and the precursor materials. Therefore, the method and apparatus described and suggested by Gurev is limited in utility to low deposition rate processes.
Accordingly, there exists a further need for an electron beam vaporized, activated reactive evaporation process in which the vaporized precursor material, as well as the reactant gases introduced into the apparatus, are excited by a radio frequency-type plasma to facilitate the reaction and/or recombination of the reactant gases and vaporized precursor material, which process is not dependent upon secondary electrons for generation and maintenance of the plasma. The instant invention fulfills these needs insofar as it provides for a plasma activated evaporation process in which the reaction rate of evaporating species is independent of the evaporation rate and/or temperature thereof, and in which the reaction of the evaporated species occurs remote from the substrate. The instant invention thus may be utilized in the preparation of thin films from an unlimited variety of materials (regardless of evaporation temperatures of the precursor materials and/or desired rates of deposition. Accordingly, the instant invention is especially well suited for the rapid deposition of thin films as a step in the fabrication of electronic devices such as integrated circuits, memory arrays, MOS transistors, photovoltaic devices and the like, without deleteriously effecting previously deposited semiconductor material or other low melting point electronic materials.
The method of the instant invention has particular utility in the preparation of photovoltaic devices such as solar cells fabricated from thin film layers of amorphous semiconductor material. Said method may be advantageously employed to deposit electrodes of a transparent electrically conductive oxide (TCO) material, such as indium tin oxide or tin oxide, upon said thin film layers of semiconductor material, such as the silicon and germanium alloys of the photovoltaic devices. The method of the instant invention may likewise be employed to deposit thin films of highly resistive materials, such as silicon oxides, silicon nitrides, cermets, etc. for use as barrier layers, protective layers, anti-reflective coatings, etc., in electronic, semiconductor and photoresponsive devices.
It should therefore be apparent that the instant invention is specifically adapted for use in the manufacture of any type of thin film device, and has special utility in the fabrication of large area photovoltaic cells which incorporate thin film layers of amorphous semiconductor alloys. Through the judicious use of the present process, the previously deposited amorphous semiconductor alloy layers are not crystallized, despite the subsequent deposition of a thin film electrode layer, which previously could only be accomplished at high temperatures.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous," as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge deposition or other vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductors Equivalent To Crystalline Semiconductors," issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374 of Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled "Amorphous Semiconductors Equivalent To Crystalline Semiconductors"; and U.S. Pat. application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled "Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy." As disclosed in these patents and application, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409 for "A Method of Making P-Doped Silicon Films"; U.S. Pat. No. 4,410,588 for "Continuous Amorphous Solar Cell Production System," and in pending patent applications: Ser. No. 44,386, filed Mar. 16, 1981, for "Continuous Systems For Depositing Amorphous Semiconductor Material"; Ser. No. 306,146, filed Sept. 28, 1981, for "Multiple Chamber Deposition And Isolation System And Method"; Ser. No. 359,825, filed Mar. 19, 1982 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells"; and Ser. No. 460,629 filed Jan. 24, 1983 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells." As disclosed in these patents and patent applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for the deposition of a p-type semiconductor alloy, the second chamber is dedicated for the deposition of an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for the deposition of an n-type semiconductor alloy.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained. It is for the production of large area photovoltaic cells fabricated from the materials and by the processes enumerated hereinabove, inter alia, that the radio frequency activated electron beam evaporation method and apparatus of the instant invention may be utilized. Employing the techniques referred to in the patents and patent applications referenced above, and the teachings of the instant invention, a novel process for depositing thin film layers as a step in the manufacture of large area photovoltaic cells is disclosed.
These and other objects and advantages of the instant invention will become clear from the drawings, the detailed decription of the invention and the claims which follow.